1. Field of the Invention
The present invention relates to a waveform controlling device for shaping the waveform of a tracking error signal used for a tracking servo control in which the irradiating position of a light beam is controlled to be placed on the position of an information track formed on an information recording medium.
2. Description of the Related Art
To optically reproduce information recorded on optical disks, a light beam must be focused onto an information track along which information is recorded, and the light beam irradiating position (light spot) must follow the information track. To this end, a focus servo control and a tracking servo control have been effected in a conventional optical disk reproduction device. The focus servo control uses a focus error signal produced from a light reflected from the optical disk and the tracking servo control uses a tracking error signal that is also produced from the reflected light from the optical disk.
Conventional tracking servo controlling methods include the 3-beam method and the push-pull method. The tracking error signal is produced when the light spot traverses the information track. FIG. 1(a) shows a waveform of the tracking error signal STE produced by the 3-beam or the push-pull method.
In effecting the tracking servo control using the tracking error signal STE, a distance of the light spot radially displaced from the information track is obtained from the tracking error signal STE, and then the position of an objective lens disposed in an optical path of the light beam is moved by means of an actuator toward the information track based on the thus obtained displaced distance of the light spot. In this manner, the radial position of the light spot is controlled typically, the waveform of the tracking error signal is symmetrical with respect to a reference level (zero level) when the light spot displaces the same distance in opposite radial directions relative to the information track.
However, this is not true in some cases with the conventional optical disk reproduction device. When the focus of the light spot is symmetrically undulating along the information track, although the tracking error signal must have a symmetrical waveform shape with respect to the zero level as shown by a dotted line in FIG. 1(b), the actual tracking error signal STE' have an asymmetrical waveform shape as shown by a solid line in FIG. 1(b). In this manner, even if the light spot is symmetrically tracking off from the information track, a tracking error signal having an asymmetrical waveform may be obtained. This is caused by the positional displacement of light emitting elements which produce a preceding beam and a post beam used for producing the tracking error signal in the 3-beam method or by the difference in the sensitivity of two photo-detectors for detecting the preceding and post beams. The light emitting elements are displaced by external vibrations imparted on the optical disk reproduction device. When the light emitting elements are positionally displaced, the light amount detecting capability of the corresponding photo-detectors does not longer remains the same. The sensitivity of the two optical detectors may become unequal due to aging. Particularly, vehicle-mounted compact disks (CDs) suffer from these causes.
When the tracking error signal is unbalanced, the tracking servo control cannot be accurately performed because a correspondence between the level of the tracking error signal and the amount of shift of the light spot from the information is shifted from that initially set.
It has been a general practice to correct the waveform of the tracking error signal using an oscilloscope when the unbalance of the tracking error signal is noted. Specifically, when the unbalance of the tracking error signal is found at the time of manufacture of the optical disk reproduction device, the correction of the tracking error signal is performed before shipment of the device. When the unbalance of the tracking error signal is found after shipment of the device, the device must be sent for repair. In either of the cases, it takes a long time to repair it, and it is particularly inconvenient for the user because of unavailability of the device during repair.
The correction of the tracking error signal has been done using a reference optical disk exclusively used for this purpose. Because this reference optical disk is not exactly the same as the optical disks subject to an actual use, the unbalance of the tracking error signal may be noted when the optical disks are reproduced even if the unbalance of the tracking error signal was corrected using the reference optical disk.